Negligible Bowing Effect of Bandgap and Lattice Constant in a Variety of Compositions using Large Tilt Distortion in a Cesium–Lead Mixed‐Halide System

Abstract Halide perovskites have broad bandgap tunability, making them suitable for diverse applications in optoelectronics and photovoltaics. The optical bandgap of halide perovskites varies almost linearly with the halide ionic size, and therefore, it can be controlled through compositional engineering. However, the mechanism underlying this low‐bandgap variation is not yet fully understood. Thus, this study comprehensively investigates the bandgap bowing of cesium–lead mixed‐halide perovskites using compositional engineering and demonstrates that bandgap bowing is extremely small in a variety of compositions including Cl, Br, and I. Subsequently, through density functional theory calculations, it is suggested that the antibonding character of the valence band maximum, tilt distortion of halide ions, and entropy effect leading to equal participation of various halide ions around Pb are collectively responsible for the small bandgap bowing.